This invention relates to sensing devices which utilize the gyroscopic principle, i.e., measuring the Coriolis force created by the conservation of momentum of a moving body. Specifically, the invention concerns devices called micro-gyros, which are small and inexpensive. They rely on conservation of momentum of a structure having limited oscillation motion, rather than full rotation. They are able to withstand rough environments for long periods of time.
In this field, the terms used to describe the directions of motions and of forces can be confusing. Applicant in describing and claiming the present invention will refer to the three separate directions (which are orthogonally related to one another) as follows: (a) the driven element, which is cause to oscillate at a predetermined, arbitrary rate inside the gyro, moves around the drive axis; (b) the velocity of the gyro environment, which is to be determined by the gyro, is around the rate axis; and (c) the Coriolis force, which is a function of the velocity of the gyro environment, is measured by motion of a sensing element around the output axis.
A number of patents have been issued to the Charles Draper Laboratory for such micro-gyro sensors, including U.S. Pat. Nos. 5,016,072; 5,203,208; 5,349,855; 5,408,877; 5,535,902; and 5,555,765. The earliest of the listed Draper patents refers to "U.S. Pat. No. 4,598,585 to Boxenhorn, which discloses a planar micro-mechanical vibratory gyroscope adapted for small geometry configurations which may be constructed using semiconductor fabrication mass production techniques".
In Draper U.S. Pat. No. 5,016,072, a single element mass is supported by a system of flexible linkages, made of semiconductor material, to allow for movement in two axes. A system of electrodes drives the mass to vibrate in one axis, and senses the motion of the mass due to Coriolis force in another axis. In another patent issued to Draper, U.S. Pat. No. 5,203,208, the same concept is extended to a symmetrical support linkage system. Draper U.S. Pat. No. 5,349,855 is another micro gyro design wherein an element mass is supported by a system of flexures. The element is driven laterally, and reacts rotationally due to Coriolis force. U.S. Pat. No. 5,408,877 issued to Draper relies on moving a single proof mass along one linear axis, and senses the motion of the same element along an orthogonal axis due to Coriolis force. Draper U.S. Pat. No. 5,555,765 shows a micro gyro using a single mass element formed into the shape of a wheel. By oscillating the wheel mass, a rotation about an axis normal to the plane of the wheel will create Coriolis force that will tilt the wheel.
U.S. Pat. No. 5,359,893 issued to Motorola uses a pair of elements supported in an "H-shaped" linkage frame so that angular velocity can be measured in two perpendicular axes. U.S. Pat. No. 5,488,862 of Neukermans et al involves a design with an outer torsional frame that is excited to tilt about an axis in the plane of the frame; an inner frame responds to the Coriolis force by oscillating (and carrying with it the outer frame) in an axis orthogonal to the outer frame axis. Both the drive and the sensing mechanisms rely on piezo-voltage actuators and sensors mounted on the hinges. In addition to the listed patents, substantial micro-gyro work has been done by the Berkeley Sensor and Actuator Center.
There are several significant defects in the prior art micro-gyros. With the exception of U.S. Pat. No. 5,488,862, reliance is on a single mass element for both driving and sensing functions. This coupling of the driving and output motion severely limits the sensitivity of the gyro. For example, as the single element is driven to vibrate, a key parameter that affects the driving mechanism is the alignment between the element and the drive electrodes. In the presence of an angular rate, the Coriolis force will create a secondary motion on the same element, thereby disturbing the alignment between the mass and the driving electrodes. Complex control schemes are necessary to compensate for such undesirable motions.
Furthermore, with only a single mass element, it is difficult to match the two resonant frequencies. Corrections are limited to support linkages only; any correction made to the mass element will alter both driving and output resonance simultaneously. Another complication is that the proximity of a single element to multiple electrodes leads to stray capacitance and coupled electric fields that are significant sources of electrical noise.
In U.S. Pat. No. 5,488,862, although two elements are used, the design does not allow for independent movement of each element. The outer frame is rigidly connected to the inner frame, so that the two frames essentially behave as a single mass element. When the inner frame rotates, the outer frame rotates with it. Another shortcoming in that design is that the outer frame has severely limited movement, due to typically very small thickness spaces (usually micro meters) in micromachining. The limited rotation of the frame results in low angular momentum, and hence low gyro sensitivity.
In all prior art designs there is also a lack of ability to electronically correct for imbalances of the system due to manufacturing tolerances. The problem is worsened by the use of a single element for drive and output motions. Since the same physical element is driven and sensed, due to mechanical coupling any forces used to balance the mass will often generate undesirable signals that corrupt the intended signal. For the same reason, self-testing of the micro gyro in prior art designs is very difficult.